Typically, variable pitch aircraft propeller systems include a plurality of propeller blades mounted for pivotal (pitch adjusting) movement about the longitudinal axes thereof to a rotary hub driven by the aircraft's engine. The hub usually includes a chamber interiorly thereof, the chamber accommodating a hydromechanical pitch actuation system including an hydraulic actuator having an output member such as a piston or the like connected to a movable pintle or cam. The cam engages a follower operably connected to a root portion of the blade. Energization of the actuator moves the cam and hence the follower, thereby pivotally adjusting the pitch of the aircraft blades to, for example, control the speed of the propeller and therefore the operation of the aircraft. Such aircraft propeller actuation systems are shown extensively in the prior art, such as for example, in U.S. Pat. No. 3,068,943 to Fischer.
It will be recognized by those skilled in the art that such hyromechanical propeller pitch actuation systems must, in the normal operation thereof, accurately perform a number of different mechanical functions. For example, the system must provide relatively precise control of the admission of hydraulic fluid to the actuator and the drainage of such fluid therefrom. The system must also provide actuation and deactuation signals to the actuator from the system's controller and provide a "pitch lock" function for maintenance of blade pitch setting in the face of a malfunction in the actuation system's hydraulics. For enhanced compactness, minimization of weight, simplicity of structure, ease in assembly and economy of manufacture, it is desirable that the mechanical functions of the actuation system noted herein be performed by as few a number of component parts as possible. Accordingly, simplification of prior art propeller pitch change actuation systems has been continually sought. Improvements in the accuracy of prior art pitch change actuation systems (the ability of the systems to maintain desired blade pitch settings determined by the system's controller) have also been sought. However, a number of characteristics inherent in such pitch change actuation systems have been inimical to the ability of the systems to accurately set and maintain blade pitch. For example, in a .beta. (pitch angle set in response to throttle lever position rather than engine speed governor operation) mode of operation, the pitch change actuation system must accurately input pitch change signals from the controller to the pitch actuator and feed signals indicative of actual blade pitch setting back to the controller. Typically, such feedback signals have been provided to the controller by such mechanisms as feedback rods, levers or other mechanical assemblies. Such mechanisms are often characterized by operational backlash due to normal manufacturing tolerances and susceptibility to thermal expansion and contraction. These backlash and thermal effects in addition to changes in length of system components due to the normal loading thereof, all detract from the accuracy with which blade pitch may be set and held by the system under beta operating conditions. Furthermore, the hydraulics associated with such systems have been difficult at best to index (calibrate) when the pitch actuation system is being assembled within the aircraft.